Beta-Adrenergic receptors are present in a number of cultured cell lines and their ability to function is altered by a number of factors in the culture medium and the length of time in culture. When astrocytoma cells (C6 glioma) undergo many generations, the ability of the cell to respond to catecholamines is profoundly reduced. This reduction in responsiveness is due to several factors including a decreased number of beta-adrenergic receptors and increased phosphodiesterase activity; this results in a 5-fold decrease in cAMP half-life in the cell. Ultimately, these findings may be important in understanding age-related phenomena. On the other hand, avian muscle cells (myoblasts) held in culture will fuse to form myotubes which respond to catecholamines. The process of fusion is correlated with the appearance of new synthesis of beta-adrenergic receptors which appear to couple with a pre-existent adenylate cyclase.